1. Field of the Invention
The present invention relates to a solid-state image pickup device, a signal processing method for the same, and an image pickup apparatus using the same, and more particularly to a solid-state image pickup device utilizing a so-called Column Analog-to-Digital Conversion (ADC) system, a signal processing method for the same, and an image pickup apparatus using the same.
2. Description of the Related Art
An amplification type solid-state image pickup device as one kind of X-Y address type solid-state image pickup device, for example, a CMOS type (including a MOS type) solid-state image pickup device (hereinafter referred to as “a CMOS image sensor”) is known as one system of solid-state image pickup devices. A technique called a column system is known in this CMOS image sensor. In the column system, column processing portions are provided independently of one another so as to correspond to pixel columns, respectively, for a pixel array portion in which pixels including respective photo-electric conversion elements are two-dimensionally disposed in a matrix. Also, signals (pixel signals) are successively read out every pixel column from the respective pixels in the pixel array portion to be temporarily held in the corresponding one of the column processing portions. Also, the pixel signals for one column are successively read out at a predetermined timing.
In addition, a column Analog-to-Digital Conversion (ADC) system with which a column processing portion is given an ADC function is known as one kind of column system. With this column ADC system, a comparator compares an analog pixel signal with a reference signal having a RAMP waveform, thereby generating a pulse signal having a size (pulse width), in a time axis direction, corresponding to a size of the pixel signal. Also, a counter counts predetermined clock signals for a time period corresponding to the pulse width of the pulse signal, and the resulting count value is set as a digital signal corresponding to the size of the pixel signal. In such a manner, the AD conversion is carried out.
In the CMOS image sensor using such a column ADC system, a so-called thinning reading method of thinning (skipping) pixel information to read out the resulting pixel information is known as a method of increasing a frame rate. However, when the thinning reading method is used, since although the electric charges are accumulated through the photoelectric conversion, there are some pixels from which the electric charges accumulated therein are discarded, the sensitivity is reduced. In order to solve a problem about the reduction of the sensitivity, heretofore, analog pixel signals outputted from respective unit pixels are converted into digital values by utilizing a column ADC system, and the resulting digital values corresponding to a plurality of pixels in a vertical direction, respectively, are added to one another to be read out. This technique, for example, is disclosed in Japanese Patent Laid-Open No. 2005-278135 (hereinafter referred to as Patent Document 1).